Chester - a non-proportional controller simply applies a pre-set voltage to the trailer every time you hit the brakes. The RedArc has a knob that you turn to adjust the voltage. That's all it does - doesn't matter if you're going 10kph in a car park or 100kph on the freeway, you get the voltage you set.
A proportional controller (like the P3) has a little g-sensor inside it. If you just tap the brakes, it senses a slight deceleration, and applies a small voltage. If you really stomp the brakes, the controller ramps the voltage right up to match the trailer's braking to the car. Easy to see how this gets you a much better result on/off the road.
Nothing wrong with a static controller, you've just got to be constantly adjusting the knob, so make sure it's within easy reach. The proportional controllers are much more of a fit-and-forget thing - set it for the weight of the trailer, and off you go.
Aside - the P3 also comes with a heap of functionality over the basic controllers, it's got a bunch of diagnostic modes, can tell you when something is wrong, etc. it's also really easy to switch between trailer profiles. It's this additional fruit that sold me on it, for the marginal extra price.
My $0.02.
Cheers!
Matto